reset: rename `reset_head()`

reset: rename `reset_head()`

In a subsequent commit we're about to adapt `reset_head()` so that the
reference update to HEAD is optional, only. At this point the function
starts to feel misnamed, as it doesn't necessarily have anything to do
with the HEAD reference anymore. The gist of the function then is that
we reset the working tree to a specific new commit, updating both the
index and the checked-out files.

Rename it to `reset_working_tree()` to better reflect that.

Note that we don't adjust the flags yet. This will happen in a
subsequent commit.

Suggested-by: Phillip Wood <redacted>
Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>